Long persistent red phosphors have been difficult to achieve. Alkaline earth sulfides such as SrS and (Zn,Cd)S:Cu are known. Lindmayer et al in U.S. Pat. No. 5,043,096 disclose that sulfides activated with lanthanum, europium and cerium are redemitting phosphors; however, they are readily decomposed when exposed to moisture. Further they are fluxed with oxides and a halide such as LiF. The fired phosphor was highly sintered and had to be ground to obtain a useful material. However, grinding degrades the emission, and even heating the ground phosphor did not fully restore its emission.
Royce et al, U.S. Pat. No. 5,650,094, disclose red phosphors of (Ca,Pr,M) titanates, wherein M is Zn and/or Mn. Although described as long emitting phosphors, the emission lasts only about 10 minutes.
The only way of obtaining long persistence red emitting materials that has been achieved so far is by filtering green emitting phosphors, such as Zn:Cu:(Co,Sr) or alkaline earth aluminates activated with europium and dysprosium. However, this method results in a decreased emission intensity.
Thus a search for long persistence, red emitting phosphors has continued.